Imaging hole transport at catalyst-coated MIS photoanodes for water splitting under high-intensity illumination

Metal-insulator-semiconductor (MIS) photoanodes are increasingly employed for solar water splitting due to their high performance. Here, we introduce a photoelectrochemical (PEC) mapping system that utilizes the scanning light beam of a confocal microscope, focused onto the surface of a photoanode. This approach enables submicrometric spatial resolution, which we employ to study photocurrent generation in MIS photoanodes constructed from n-type Si (n-Si) coated with oxygen evolution reaction (OER)-active Ni micropatterns. Our study highlights several key features. First, minority carrier (hole) transport beneath the uncoated SiO _x surface is influenced by the presence of electrolyte at the interface. Second, the main photocurrent contribution arises from illumination of Ni-free regions, even mm away from the Ni active sites. Third, the hole collection at the catalyst becomes significantly limited under high-intensity illumination regimes. These results are rationalized, allowing a general description of the transport of photogenerated holes in these MIS photoanodes. Furthermore, PEC mapping directly reveals that the issue of limited hole collection at high illumination intensities can be mitigated by shortening the spacing between catalyst islands. These findings offer key insights for designing more efficient PEC-based solar fuel systems that perform efficiently under high light intensities, such as concentrated sunlight.